Aircraft gas turbine engines sometimes use hollow fan blades to effect a weight saving. These blades generally have longitudinal ribs inside the blade to stiffen the blade with these ribs extending through from the pressure side to the suction of the blade.
A method of forming such blades is described in U.S. Pat. No. 5,063,662 issued to Porter et al. During this process the blade interior is purged with argon gas to remove all water vapor. Openings at alternate ends of alternate ribs are provided to that a sinuous flowpath is formed between the ribs permitting the purge gas flow through the entire blade. A sufficient flow rate must be maintained to avoid stagnant pockets which would not be cleared.
The fan blades of a gas turbine engine must be designed for consideration of foreign object damage. This can occur as a result of bird strikes or because of ingestion of foreign material from the ground. Cordwise ribs therefore are added with sections between the longitudinal ribs. The placement of these cordwise rib sections is determined by the strength requirement of particular locations. Such ribs are at locations which would block the purge flow through the blade.
Semi-circular cuts of each half prior to diffusion bonding the two halves together have been used. During an object impact bending around a radial axis occurs, with the pressure side of the blade going into tension and the suction side going into compression. At the hole locations the cordwise ribs have been found to buckle. This buckling has produced sufficient strain to cause local cracking of the cordwise rib.
The use of a semi-circular cut on only one half of the blade has produced too restrictive a final opening. Material from the uncut side flows up into the opening.